1. Technical Field
The present invention relates to a thin-film circuit device (thin-film semiconductor device) including a thin-film circuit layer, disposed on a flexible substrate, including semiconductor elements; a method for manufacturing such a thin-film circuit device; and an electronic apparatus including such a thin-film circuit device.
2. Related Art
Thin-film circuit devices include substrates; thin-film circuit layers, disposed on the substrates, including semiconductor elements; and other components. The substrates are usually prepared from single-crystal silicon wafers, quartz glass wafers, heat-resistant glass wafers, resin films, or the like. These wafers are selected depending on the required performance and functions of the thin-film circuit devices. In particular, the resin films are preferable because a substrate prepared from a resin film is thin and flexible and therefore a thin-film circuit device including the substrate is light-weighted and flexible.
Examples of methods for manufacturing thin-film circuit devices including substrates prepared from resin films include a method in which a semiconductor layer, an insulating layer, a metal layer, and the like are deposited on a resin film in that order and a method in which a thin-film circuit layer is formed on a heat-resistant substrate, released from the heat-resistant substrate, and then joined to a resin film (a release transfer process). Japanese Unexamined Patent Application Publication No. 10-125931 (hereinafter referred to as Patent Document 1) discloses a method for manufacturing a thin-film circuit device by a release transfer process.
A thin-film circuit device including a substrate prepared from a resin film can have defects due to a difference in physical property between the substrate and a thin-film circuit layer.
The thin-film circuit layer usually includes an inorganic thin-film formed by depositing an inorganic material on the substrate by a chemical vapor deposition (CVD) process or a sputtering process. The inorganic thin-film has a large elastic constant of several ten gigapascals and a small linear expansion coefficient of several to 10 or more ppm/K. On the other hand, the resin film has a large linear expansion coefficient of about 10 to 50 ppm/K.
In the thin-film circuit device, since such materials having different physical properties are joined to each other, a change in temperature causes thermal stresses in the substrate and the thin-film circuit layer because of the difference in linear expansion coefficient. Since the thin-film circuit layer usually has a small thickness of several micrometers and a small cross-sectional area, a large thermal stress is developed in the thin-film circuit layer. If the thermal stress exceeds the breaking strength of the thin-film circuit layer, the thin-film circuit layer is broken. This causes the failure of the thin-film circuit device.
When the substrate is distorted or bent, a bending stress is developed in the thin-film circuit layer. Even a small strain causes a large stress in the thin-film circuit layer because the thin-film circuit layer has a large elastic constant of several ten gigapascals. Therefore, the stress caused by distortion such as bending can break the thin-film circuit layer.
End portions of the thin-film circuit device can have fine cracks or notches formed in a cutting step in which the thin-film circuit device is separated from other thin-film circuit devices. Large stresses are developed in the end portions thereof because of stress concentration; hence, the end portions thereof serve as weak points that may cause the breakage of the thin-film circuit device.